Because of the toxicity of elemental mercury and environmentally unstable forms of mercury, many environmental regulatory agencies, such as the U.S. Environmental Protection Agency, restrict their disposal and release. While the disposal and release of elemental mercury and many unstable forms of mercury remains a problem, hazardous waste containing such materials are being generated daily by many sources in many forms. One form of potentially hazardous mercury is mercury-containing gas streams, such as off gases or purge gas from various industrial processes.
One source of mercury-containing gas streams is from various processes involved in the extraction and refining of gold. A well documented phenomenon is the affinity of gold for mercury, a fact that has been used in the extraction and refining of gold from ores for many centuries. This natural attraction also means that mercury is often found in gold deposits and is collected and concentrated along with the gold during the extraction and refining process. The mercury is separated from the gold during the refinement process and either is sold as a byproduct or stabilized for storage on-site.
Other sources of mercury-containing gas streams include flue streams from coal-fired and oil-fired power plants, off gas streams from sewage sludge, municipal waste and medical waste incinerators, chlor-alkali plants, Portland Cement Processing, and pulp and paper manufacturing.
Several processes have been employed to remove mercury from a gas stream. One such process is known as a regenerable mercury control system, which exploits a noble metal sorbent to capture both elemental and oxidized forms of mercury. Once saturated with mercury, the sorbent is regenerated and can be re-used to collect mercury from a gas stream. Regenerable noble metal sorbents have been tested on laboratory and pilot-scales, but not on an industry scale. Capital costs for a noble metal sorbent system may not be cost competitive with other mercury control technologies. Degradation in mercury capacity have been noted when noble metal sorbents are used in an oxidizing environment, such as controlling mercury emissions from coal-fired flue gas streams.
Another process has been tested in which the mercury-containing gas stream is introduced into a reactor containing a sulfide solution, such as calcium polysulfide. Mercury in the gas stream would react directly with the polysulfide when oxygen was present in the gas stream. However, the process produces calcium sulfate and mercuric sulfide precipitates, which tend to accumulate in the reactor and eventually cause extreme operational problems.
Another process to remove and stabilize mercury from a gas stream requires the condensation of mercury vapor at extremely low temperatures, leading to the simultaneous condensation of water or the formation of ice, depending on the temperature of the condenser. This creates additional problems because elemental mercury is at least slightly soluble in water and therefore any wastewater from the process must be treated as hazardous.
As such, a need exists for an improved process to remove vapor phase mercury and mercury-containing compounds from a gas stream and to stablilize the reaction product into a solid, stable compound that can be disposed of as a non-hazardous waste.